Description of the Prior Art
It has been common practice to use signalling lamps or lights at airports, street intersections, and/or railroads, etc. to provide information on an around-the-clock basis. Typically, three different colored signals are displayed i.e., red, yellow and green. A red signal indicated that the vehicle should stop and a yellow signal meant that the vehicle may proceed with caution. A green signal was used to inform a person or vehicle that it was safe to proceed.
The light density of the aformentioned three colored signals must be of sufficient quality so that the distinguishing color will be visible at great distances i.e., a railroad signal would have to be seen at distances greater than one mile. Reflectors were used to collimate the lamp's light so that the signal would be able to be seen at a distance that would enable a person or vehicle to stop without being injured. One of the disadvantages of the prior art was that an outside light source, such as direct sunlight or a car's headlights may produce a phantom signal by being directed onto one of the three colored signals and being reflected back out. This phantom signal could cause a potentially dangerous situation i.e., a person may be of the opinion that the signalling device is producing a green signal when the signalling device is not producing a green signal.
The prior art used one lamp for each color or one lamp and a moveable lens mechanism to produce a signal having three aspects i.e., red, yellow and green. The moveable lens mechanism was controlled by a complex series of relays which allowed the selection of the desired aspect. When no energy was applied to the relay system, the lens mechanism remained in a neutral position and a red aspect was produced. When energy of one polarity was applied to the relay system, the lens mechanism would move to produce a yellow aspect and when energy of the opposite polarity was applied to the relay system the lens mechanism would produce a green aspect.
One of the disadvantages of the single lens and moveable relay lens system was that if a vandal shot a projectile through the lamp, the lenses could be locked in one position and the signalling device would only be capable of producing one colored signal. The foregoing could lead to a potentially hazardous situation. Other disadvantages of the single lens and moveable lens system are that the relay mechanism is expensive and difficult to maintain.